This invention relates to overbased metal salts, and more particularly to an improved process for preparing overbased calcium sulfonates which are used as detergent and reserve alkalinity lubricating oil additives.
In the course of operation, internal combustion engines convert lubricating oil to acidic degradation products. Those acidic degradation products attack and corrode engine parts and catalyze the formation of sludge, thereby reducing lubricity and accelerating wear of moving parts in contact with the lubricating oil.
It is desirable to add basic substances to the lubricating oil which neutralize acids as they are formed in the engine before they reach concentrations sufficient to cause corrosion or to catalyze the sludge formation. Adding an alkalinity agent to the detergent in motor oil is known as overbasing. Colloidal carbonates of the alkaline earth metals have been found to be well suited for this purpose. These carbonate dispersions are stabilized by oil soluble surface active agents such as the sulfonates of the alkaline earth metals in which the sulfonic acid portion of the molecule has a molecular weight of preferably 350 to 600. The sulfonates are generally made by sulfonation of lubricating oil fractions from petroleum and/or by sulfonation of alkyl benzenes having the desired molecular weight for this purpose. Benzene alkylates with straight chain alkyl groups are especially desirable.
There is an increasing demand for an overbased sulfonate having at least a 500 TBN in the marine lubricant product market. Because of the higher TBN content, less dosage of this product is required in the additive treatment to obtain equivalent or better diesel engine performance. The current sulfonate overbasing process cannot produce an acceptable 500 TBN overbased sulfonate, because it produces either an insufficiently overbased, too viscous, or base oil-incompatible product. By modifying the current sulfonate overbasing process, a cost-effective improved process can make a fluid and base oil-compatible 500 TBN overbased calcium sulfonate that provides an effective protection to marine diesel engines.
In general, the process of preparing oils which contain overbased calcium sulfonates comprises reacting a solution of alkarylbenzene sulfonic acids having a molecular weight greater than 400, in oil with calcium oxide or hydroxide and bubbling carbon dioxide through the reaction mixture; thereby incorporating an excess of calcium carbonate into the calcium sulfonate which confers reserve alkalinity to the product.
Thus, it is an object of the present invention to provide a method of producing effective overbased calcium sulfonates that contain only amorphous calcium carbonate and have a TBN of at least 500.